The present invention relates to a method, for use with a scanning radar, for detecting a stationary object such as a bridge, a road sign, a billboard, or the like located above the road ahead or on the roadside.
For vehicle-to-vehicle distance control, a vehicle-mounted radar system is used which projects a radar beam forward and thereby detects an object such as a vehicle located in the path ahead. Such radar systems include systems that use radio waves such as millimeter waves and systems that use laser light. Using such a radar system, the distance and relative velocity with respect to the vehicle ahead and the accurate position of the vehicle ahead are detected to control the vehicle-to-vehicle distance.
A scanning radar scans a radar beam by moving the beam from left to right or from right to left with small step angles within a predetermined time. At each step angle, the radar-equipped vehicle projects a radar beam toward the vehicle traveling on the road ahead, and the reflected wave from the vehicle ahead is received and processed to detect the presence of the vehicle ahead and compute the distance and relative velocity with respect to that vehicle.
However, in the case of conventional scanning radar, for example, FM-CW radar, the vertical beam width of an antenna transmitting a radar signal is about 4.0 degrees. As a result, a structure such as a road sign located far away from the radar-equipped vehicle or a bridge located above the road ahead may also be detected as a target object in vehicle-to-vehicle control, etc. because such structures cannot be distinguished from obstacles such as a vehicle stopped on the road ahead or a structure placed on the road for construction work.
It is an object of the present invention to provide a method for distinguishing a stationary object, such as a road sign, a billboard, a bridge, or like structure located above the road ahead or on the roadside, from a vehicle located in the path of a radar-equipped vehicle.
According to a stationary object detection method for a scanning radar pursuant to the present invention, of the peaks generated based on a radar signal reflected from a target, peaks having substantially the same frequency are grouped together. If the target is a bridge or a road sign, the target is not detected unless it is located at some distance away from the radar-equipped vehicle; in view of this, first a decision is made as to whether or not the frequency of the grouped peaks is equal to/higher than a predetermined value, that is, whether or not the distance to the target is equal to/greater than a predetermined value. If it is equal to/greater than the predetermined value, then a decision is made as to whether or not the number of grouped peaks is equal to/greater than a predetermined number. In the case of a bridge or a road sign, since the structure usually has a horizontally spreading shape, many peaks are detected, with the peaks distributed over a wide angle range. Accordingly, if the number of peaks is equal to/greater than the predetermined number, it is decided that the target is an overhead bridge candidate or an overhead bridge, that is, a bridge, a road sign, a billboard, or like structure located above the road ahead or on the roadside ahead.
Then, pairing is applied to peaks signals obtained from the target determined as the overhead bridge candidate, to detect a relative velocity with respect to the target, and if the relative velocity is substantially equal to the speed of the radar-equipped vehicle, it is decided that the target is an overhead bridge.
Further, if the target determined as the overhead bridge candidate continues to be detected when the target comes within a prescribed distance, the decision that the target is an overhead bridge is undone.
The method of the present invention further comprises the step of taking over previous data including a flag that is set to indicate that the target is an overhead bridge candidate.
According to the present invention, when the target located in the path ahead is a stationary object, as the method of the invention can identify whether the target is a vehicle stopping on the road ahead or an overhead bridge, such as a bridge, a road sign, or a billboard, located above the road ahead or on the roadside ahead, and can therefore determine whether the target is one that is to be treated as a control target in vehicle-to-vehicle control, etc., accurate vehicle control can be accomplished.
Furthermore, according to the method of the invention, the decision as to whether the target is an overhead bridge or not can be made using a simple method such as detecting the spreading of peaks having the same frequency and determining whether the position detected is at least at a prescribed distance away.